Conjugated polymer semiconductors can be processed from solution, which makes these materials compatible with a range of printing techniques and enables low-cost manufacturing of electronic devices on light-weight, flexible substrates. A particularly attractive feature of solution processing is the ease of forming new materials with designed properties that comprise a mixture of several polymer components. By carefully controlling the morphology of a polymer blend deposited from a suitable cosolvent through judicious choice of composition and processing conditions the mechanical, optical and electronic properties of the resulting film can often be improved compared to a single-component system (Morteani et al., Phys. Rev. Lett. 92, 247402 (2004)); Halls et al., Nature 376, 498 (1995)). In some cases the addition of a suitable binder polymer is even imperative, for example to meet the ink rheology requirements for specific printing steps (de Gans, Adv. Mat. 16, 203 (2004)). In the case of a polymer FET the use of binders is challenging because in a two-component active layer, charge transport tends to occur in one of the components preferentially, for example, hole charge carriers are located preferentially in the component with the lowest ionisation potential, and the presence of a second semiconducting or dielectric component tends to degrade device performance. This has been observed in several systems. In blends of poly(3-hexylthiophene) (P3HT) with polystyrene (PS) the mobility decreases by several orders of magnitude with decreasing percentage of P3HT (Babel, et al. Macromolecules 37, 9835 (2004)). In P3HT:PS diblock copolymers the conductivity is much reduced when the content of the P3HT block in the overall BCP is decreased (Liu, et al., Angewandte Chemie International Edition 41, 329 (2002)). The morphology of polymer blends is controlled by phase separation which occurs even for small unfavourable enthalpic interactions between the two polymers as a result of the small entropy of mixing. Under most conditions phase separation proceeds laterally in the plane of the film. Vertically stratified morphologies are less common (Arias et al., Macromolecules 34, 6005 (2001)), but are expected to be advantageous for FET applications, if the active component can be made to phase separate to the interface with the gate dielectric and create a continuous pathway for charge transport (Chua, et al., Advanced Materials 16, 1609 (2004)). It has recently been shown that a vertically stratified morphology is an intermediate morphology during spin coating, but breaks up into a laterally phase separated morphology driven by a thin film instability (Heriot et al., Nature 4, 782 (2005)). Phase separation also determines the morphology of block copolymers, but in this case it can only occur on the nanometer scale. Much of the work in the literature on polymer blends and block copolymers in the literature has been on amorphous-amorphous or crystalline-amorphous systems. Much less is known about systems in which the binder is a crystalline polymer. For conventional (non-electroactive) polymers crystalline-crystalline blends have been studied and the effect of adjusting the process temperature with respect to the melting points of the two component polymers (Ikehara, et al. Macromolecules 38, 5104 (2005)). The microstructure of conventional crystalline-crystalline blockcopolymer has also been investigated, and the influence of epitaxy or directional solvent crystallization (Park et al., Polymer 44, 6725 (2003)) has been studied.
In WO2005104625 a method is described for melt processing of an organic semiconductor. A dewetting agent in the form of a polymer is mixed into a solution of the organic semiconductor. The organic semiconductor is preferably a small molecule semiconductor, such as rubrene, which has a tendency to dewet from the substrate when heated above its melting point. This is prevented by the presence of the dewetting agent. The dewetting agent is preferably a polymer, such as polystyrene. A second substance might also be added to the formulation of the organic semiconductor to reduce the melting point and to prevent crystallization of the organic semiconductor in the as-deposited film. It is present in a composition of 1% to 50%, most preferably 5-15%. If the organic semiconductor is diluted further its charge transport properties are degraded. No mention is made of any requirements on the binder polymer to avoid this degradation.
In US 2004/0038459 a method is described for mixing an organic semiconductor with a polymer binder. The requirement for the polymer binder is that it has a low dielectric constant. However, as can be seen from the Table included in US 2004/0038459, the binder is present in the formulation at a weight ratio of typically less than 50%, and already in this range a significant reduction of field-effect mobility is observed compared to the pure semiconducting polymer. This document indicates that one criterion for selecting the binder is that its dielectric constant ∈ needs to be sufficiently low to avoid lowering of the mobility as a result of increased dipolar disorder (for PS ∈=2.6). However, no mention is made of any requirements on the binder polymer to avoid the degradation in the field-effect mobility that is observed when the semiconducting component is being diluted with the binder.